At the present time, in chemotherapy for cancer, "acquired resistance" where the cancer suppressing effect is lost during treatment is becoming a problem. Multidrug resistance where resistance is exhibited against various types of anti-neoplastic agents is becoming an important issue. As a method for overcoming this multidrug resistance, it has been reported that co-administration of the anti-neoplastic agent and some calcium antagonists (1,4-dihydropyridine compounds such as Nicardipine) is effective. (Cancer Res., 41, 1967-1972 (1981), Cancer and Chemotherapy, vol. 11, 750-759 (1984)).
Further, Japanese Unexamined Patent Publication (Kokai) No. 2-40383, Japanese Unexamined Patent Publication (Kokai) No. 2-240081, Japanese Examined Patent Publication (Kokoku) No. 6-92391, Japanese Examined Patent Publication (Kokoku) No. 6-92401, Japanese Unexamined Patent Publication (Kokai) No. 5-117235, and Japanese Unexamined Patent Publication (Kokai) No. 2-138221 describe that a 1,4-dihydropyridine compound has an action overcoming resistance to anti-neoplastic agents. Further, recently, there are reports on quinoline or indole derivatives having a similar action in overcoming resistance to anti-neoplastic agents (see Japanese Unexamined Patent Publication (Kokai) No. 4-235983 and Japanese Unexamined Patent Publication (Kokai) No. 5-43544).
On the other hand, as imidazothiazole derivatives, 2-phenylimidazo2,1-b!benzothiazole derivatives have an immunomodulating action, and therefore, patent applications have been filed as an anti-allergenic, an anti-rheumatic, and an anti-asthmatic drugs (see Japanese Unexamined Patent Publication (Kokai) No. 56-138196, Japanese Unexamined Patent Publication (Kokai) No. 57-40492, and Japanese Unexamined Patent Publication (Kokai) No. 57-149288). Further, patent applications have been filed for 2-methylimidazo2,1-b!benzothiazole and 5,6,7,8-tetrahydro-2-methylimidazo2,1-b!benzothiazole derivatives as an anti-neoplastic agent or brain enhancers (see Japanese Unexamined Patent Publication (Kokai) No. 1-319487, Japanese Unexamined Patent Publication (Kokai) No. 1-319488, Japanese Unexamined Patent Publication (Kokai) No. 2-178289, Japanese Unexamined Patent Publication (Kokai) No. 2-306918, and Japanese Unexamined Patent Publication (Kokai) No. 2-306917).
Further, as imidazothiazole derivatives, 2,3,5,6-tetrahydro-6-phenylimidazo2,1-b!thiazole (Tetramisole) is known to exhibit an immunosuppressive action and an anticancer action (see U.S. Pat. No. 3,579,530 (1971)).
However, no report has been made concerning the action of a tetrahydroimidazobenzothiazole or imidazobenzothiazole compound in overcoming resistance to anti-neoplastic agents.